1. Field of the Invention
The present invention relates to a method for measuring a steering angle of steering shaft of vehicle, more specially to a method for measuring a steering angle of steering shaft of vehicle, in which the steering angle of the steering shaft is measured by using two rotors rotated at a determined RPM ratio according to the rotation of the steering shaft.
2. Description of the Related Art
When measuring the absolute steering angle of the steering shaft, it is not easy and problematic to measure it just by means of an angle sensor, because its measurement range exceeds 360°.
Besides, when starting a vehicle, the steering angle of the steering shaft has to be measured even though the shaft is located at any angular position.
The method for measuring the absolute steering angle of the steering shaft is already known e.g. in U.S. Pat. No. 5,930,905 and U.S. Pat. No. 6,466,889B1, in which the steering angle is obtained by measuring the rotation angle of first and second rotor rotating at a constant RPM ratio according to the rotation of the steering shaft.
In such arts according to above disclosure, the absolute rotation angle of the first and second rotor can be represented respectively as ψ=ψ′+iΩ and θ=θ′+jΩ, (wherein, Ω is the measurement range of the angle sensor for measuring Ψ′ and θ′, i represents a periodic number of the first rotor as an integer indicating the number in which the absolute rotation angle of the first rotor Ψ exceeds the Ω, and also j represents a periodic number of the second rotor), and for both above American patents, the absolute steering angle Φ is obtained from the determined process where both Ψ′ and θ′ are measured and the measured values are calculated.
According to the U.S. Pat. No. 5,930,905, the Ψ′ and θ′ are measured, the measured values are applied to the specific formula (1), as follows, derived from the mutual geometric relation between Ψ, θ and Φ, and then an integer k is obtained by rounding off the value resulting from the formula (1). After that, k, Ψ′ and θ′ are applied to the following formula (2), and accordingly, the value of Φ is obtained.
                    k        =                                                            (                                  m                  +                  1                                )                            ⁢                              Θ                ′                                      -                          m              ⁢                                                          ⁢                              Ψ                ′                                              Ω                                    (        1        )                                Φ        =                                            m              ⁢                                                          ⁢                              Ψ                ′                                      +                                          (                                  m                  +                                )                            ⁢                              Θ                ′                                      -                                          (                                                      2                    ⁢                    m                                    +                  1                                )                            ⁢              k              ⁢                                                          ⁢              Ω                                            2            ⁢            n                                              (        2        )            (wherein, m is gear tooth number of the first rotor, m+1 is that of the second rotor, and n is that formed on the steering shaft. This steering shaft is geared with the first and the second rotor.)
Meanwhile, according to the U.S. Pat. No. 6,466,889B1, the steering angle Φ is obtained by using the relation between the difference Ψ−θ of the absolute rotation angles of two rotors and the periodic number i of the first rotor (it can be the second rotor), and by directly obtaining the i. Herein, if Ψ′−θ′ gained by measurement is negative number, Ψ−θ is obtained by adding Ω thereto, otherwise, the value of Ψ−θ is used as it is. After that, the i is calculated from the relation between Ψ−θ and i, and the absolute steering angle Φ of the steering shaft is obtained by using Ψ calculated from Ψ′ and i.
At this point, when the steering shaft rotates to the maximum and the i comes to k1, the difference Ψ−θ of the rotation angle has to be equal to or smaller than Ω because the measurement range of the angle sensor is Ω. (However, according to the U.S. Pat. No. 6,466,889B1, it is only equal to Ω.). Namely, until the steering shaft rotates to the maximum, the difference Ψ−θ of the rotation angles changes from 0° to Ω continuously, and i does from 0 to k1 stepwise.
Herein, in such an art according to the U.S. Pat. No. 6,466,889B1, when the difference Ψ−θ of the rotation angles changes from 0° to Ω continuously, it is assumed that i changes from 0 to k1 continuously and a linear proportional relation is established, and the i is obtained by taking the maximum integer smaller than the value which is gained by multiplying Ψ−θ derived from the measured values of two rotation angles by k1/Ω. For example, if the value obtained by multiplying Ψ−θ by k1/Ω is 5.9 . . . , the i comes to 5.
Accordingly, for the method according to the U.S. Pat. No. 6,466,889B1, i−j has to be always 0 or 1 and cannot exceed 2, because the maximum of Ψ−θ cannot be greater than Ω.